1. Field of the Invention
The present invention relates to an automatic knee control circuit for use with a television camera or the like.
2. Description of the Prior Art
Conventionally, in order to cope with a wide variety of the change of the amount of incident light, a television camera utilizes a white level compressing circuit which might be called a knee circuit.
A conventional knee circuit 10 is shown, for example, in FIG. 1. As shown in FIG. 1, a resistor 13 is interposed between an input terminal 11 to which a video signal from a camera section (not shown) is supplied and an output terminal 12, and a diode 14, a resistor 15 and a reference voltage source 16 are connected in series between the output terminal 12 and ground.
As shown in FIG. 2, an input and output characteristic of this knee circuit is linear up to a predetermined input level (knee point) Vk. When an input signal whose level is beyond the knee point Vk is supplied to this knee circuit 10, the diode 15 is turned on. The output signal is compressed on the basis of a ratio between resistance values of the two resistors 13 and 15 so that the change of the output level becomes gentle as shown by the straight line L. The inclination of the straight line L is what might be called a knee slope.
In the knee circuit 10 shown in FIG. 1, the reference voltage Vr of the reference voltage source 16 is set so that the range of the change of the incident light amount falls within, for example, 300%. Thus, the output level will exceed a predetermined upper limit value if the incident light amount in a wider variety such as, for example, 600%, and as a result, the succeeding circuit cannot cope with such change of incident light amount.
Therefore, the assignee of the present application has previously proposed an automatic knee control circuit which can automatically control a reference voltage in response to an input level (see Japanese Laid-Open Patent Gazette No. 61-46675).
In this automatic knee control circuit 10C, as shown in FIG. 3, respective input color signals R, G and B applied to terminals 11R, 11G and 11B are supplied through resistors 13R, 13G and 13B to a non-additive mixing circuit (NAM) circuit 21 in a control circuit 20, from which the signal of the highest level of the three signals is supplied to a peak detecting circuit 22. An output of this peak detecting circuit 22 is supplied through an amplifier 23 to a Junction P among resistors 15R, 15G, 15B as a reference voltage.
In the automatic knee control circuit 10C of FIG. 3, when the input level is high, the knee point is decreased, while when the input level is low, the knee point is increased.
Further, an automatic knee control circuit 30 shown in FIG. 4 also is known.
In the automatic knee control circuit 30 of FIG. 4, knee point setting circuits 33R, 33G, 33B and knee slope setting circuits 34R, 34G, 34B are connected in series between three sets of input terminals 31R, 31G, 31B and output terminals 32R, 32G, 32B of the knee circuit 30, respectively.
Outputs of the knee slope setting circuits 34R, 34G and 34B are all supplied to a non-additive mixing (NAM) circuit 41 of a control circuit 40, and the signal of the highest level in the three signals is supplied from the NAM circuit 41 to a peak hold circuit 42. An output from this peak hold circuit 42 is supplied to a comparator 43, in which it is compared with a voltage V44 of a variable voltage source 44. An output of the comparator 43 is commonly supplied to the respective knee point setting circuits 33R, 33G, 33B or the knee slope setting circuits 34R, 34G, 34B.
In the automatic knee control circuit 30 of FIG. 4, the knee point or the knee slope is automatically controlled in response to the high or the low of the input level and the variable reference voltage V44 of the variable voltage source 44.
Incidentally, with regard to the prior art of FIG. 4, when a picture is produced, it is frequently observed that the knee point and knee slope are both properly controlled in response to the user's intention in the picture production.
However, in the earlier-noted conventional automatic knee control circuits, e.g., as in FIGS. 1 and 3, only either one of the knee point and the knee slope is controlled and the other characteristic is not changed, i.e., it is previously set so that only a standardized picture can be obtained. Consequently, a proper signal processing state corresponding to the user's particular production intention cannot always be set.